The present invention relates to a hydraulic timing chain tensioner assembly including a spring-controlled oil pressure relief structure configured to vent highly pressurized oil to reduce maximum loads experienced by the timing chain.
FIG. 1 shows a typical prior art chain-driven dual overhead cam layout, including a timing chain 10 which is driven by a crankshaft 12 to rotate intake and exhaust cam shafts 14, 16, respectively. The chain 10 is guided by guide assemblies 18, 20, and a chain tensioner assembly 22 is provided for properly tensioning the chain 10. The chain tensioner assembly 22 includes a shoe (or lever) 24 which is pivotable about a fixed pivot point 26, and includes a contact feature (or button) 28. The hydraulic tensioner 30 pushes against the contact feature 28 to tension the chain 10 and to cushion loads of the chain 10 which are applied through the shoe 24.
A typical hydraulic tensioner includes a spring-loaded piston which bears against the contact feature 28 on the pivoting shoe 24. Engine oil pressure is fed through a one-way ball check valve which applies a pressure-dependent load to the piston and shoe. The tensioner also includes a coarse-stepped ratchet device to prevent total collapse of the tensioner in the absence of oil pressure.
Under certain circumstances, the chain may experience undesirably high loads as a result of the ratchet assembly in the above-described hydraulic tensioner 30. For example, in the event of a cold start of the engine, maximum oil pressure exists, so the piston extends as far as the chain linkage will allow it. The engine then heats up and the ratcheted piston must retract as the chain becomes taut due to engine block thermal expansion. However, the ratchet may then bottom out and limit back-travel of the piston. When the piston bottoms out against the ratchet, the chain may experience extremely high loads which can compromise performance and durability of the chain and increase cam drive noise.
It is therefore desirable to provide an improved hydraulic timing chain tensioner assembly in which maximum loads experienced by the chain are regulated to desirable levels.
The present invention overcomes the above-referenced shortcomings of prior art hydraulic tensioners by providing a hydraulic tensioner which includes a spring-controlled oil pressure relief structure which is operative to vent oil from the tensioner when the oil is pressurized above a predetermined level, thereby reducing maximum loads experienced by the timing chain.
More specifically, the invention provides a hydraulic timing chain tensioner assembly including a shoe configured to contact a timing chain. A hydraulic tensioner includes a plunger with a distal end abutting the shoe. The plunger is movable within an opening formed in a tensioner body. The opening includes oil and a spring member therein to cushion chain loads transmitted through the shoe and plunger. A spring-controlled oil pressure relief structure is operatively associated with the opening to vent the oil when the oil is pressurized above a predetermined level by movement of the plunger as a result of loads transmitted from the chain, thereby enabling further movement of the plunger into the opening to reduce maximum loads experienced by the timing chain.
In one embodiment, the oil pressure relief structure is a pressure relief valve in fluid communication with the opening for selectively venting the oil. The pressure relief valve includes a ball spring-loaded toward a valve seat, such that the predetermined level of oil pressure causes movement of the ball away from the seat against the spring load to open the valve and vent the pressurized oil.
In another embodiment, the oil pressure relief structure is a poppet valve structure formed by the plunger. The plunger includes a plunger sleeve, a shoe contact end, a valve head end and an internal spring compressed between the shoe contact end and a rim of the plunger sleeve. The rim is also in contact with the oil and the valve head end to define a poppet valve. The poppet valve opens when force from pressure of the oil and force from the spring member against the rim overcome force of the internal spring against the rim.
Accordingly, an object of the present invention is to provide an improved hydraulic timing chain tensioner in which maximum loads experienced by the timing chain are regulated.
Another object of the invention is to provide an improved to hydraulic tensioner having a plunger movable against an oil chamber to pressurize the oil, wherein the tensioner includes a spring-controlled oil pressure relief structure which selectively releases highly pressurized oil from the oil chamber to allow further plunger movement and reduce maximum loads on the timing chain.